Solid immersion lenses (SILs) are used in semiconductor failure analysis to increase the resolution and signal collection of optical microscopes for backside photon-based imaging and failure analysis techniques. The improvement in resolution allows optical techniques to be extended to the most modern devices, where the features are sufficiently small that they cannot be imaged without an immersion lens. These lenses are currently incorporated into commercial failure analysis equipment.
When imaging with an immersion lens many factors lead to aberrations, such as off-axis imaging, errors in fabrication of the surfaces, and an unknown thickness of the substrate and distance from back surfaces to transistor or other device level. Since spherical aberration free imaging is only possible if the region of interest sits at the center or aplanatic point of the SIL, any mismatch between the design thickness or other variable and the actual thickness or other variable of the substrate or other element in the light path will rapidly cause an increase in the focus spot size.